1. Field of the Invention
This invention relates to transmitters, receivers, and data communications systems and methods to communicate with the use of impulse-based UWB (Ultra Wide Band) signals.
2. Description of the Related Art
In the spread spectrum radio communications (Spread Spectrum), there has been proposed the technique for diversifying the correlation characteristics in such a manner that the PN (Pseudorandom Noise) code used in the DS (Direct Sequence) spread system is converted into the RZ (Return to Zero) form and multiplied by the data (refer to Japanese Patent Application Publication No. 4-347943). By converting the PN code to the RZ form, outputs of the PN codes become zero only for a given period, as shown in FIG. 1. The PN codes include both positive and negative values, so the conversion to the RZ form results in three values, positive, negative, and zero.
In recent years, attention has been focused on the UWB-IR (UWB-Impulse Radio) system that enables the large-capacity transmission and sustains multiple users. The UWB-IR system uses an impulse signal equal to or smaller than one nanosecond. The UWB-IR signals are distributed over several GHz frequencies, and can share the frequency bands without interfering with the conventional radio systems.
FIG. 2 shows a configuration of a transmitter in which the data is multiplied by the PN code converted into the RZ form and the impulse sequence. The data is multiplied by the PN code in the RZ form in a first multiplier 4, and then the data is multiplied by an impulse sequence generated in an impulse generator 3.
FIG. 3 shows signal waveforms generated in the transmitter shown in FIG. 2. FIG. 3(B) shows the signal waveform of the PN code. FIG. 3(C) shows the signal waveform of the PN code in the RZ form. FIG. 3(D) shows the signal waveform after the data shown in FIG. 3(A) is multiplied by the PN code in the RZ form and then spread. FIG. 3(E) shows the signal waveform after the spread data shown in FIG. 3(D) is multiplied by the impulse sequence and converted into the impulse radio signals. FIG. 3(F) shows the signal waveform at a receiving end.
The receiving end, however, cannot specify the waveform, when receiving and demodulating the impulse radio signals shown in FIG. 3(E). For, example, if ‘1’ continues in the spread data shown as dotted line in FIG. 3(D), the receiving end cannot specify the signal waveform (namely, the signal shown in FIG. 3(D) in which the spread code is multiplied by the impulse radio signal. This is because when the signal multiplied by the spread code is estimated with the impulse radio signal, multiple signal patterns can be speculated as shown in FIG. 3(G). Accordingly, it is impossible to specify the signal waveform.